pH Probe Calibration in COLD Weather

We are based in Michigan and with the winter months fast approaching the weather is getting cold.
We have several pH probes that are outdoors and require weekly calibrations. Our current sensors are rated to 0 C and this is the temp most mornings right now. This does not affect the process as it is always warm but it does create an issue when they are calibrated. Does anyone have any suggestions on how to keep the calibration (buffer) solution warm.